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OPA2377: When OPA2377 is designed as a follower and input is -0.8VDC, why output is zero?

Ruixin Feng
Intellectual 2215 points
Part Number: OPA2377
Other Parts Discussed in Thread: LM358

Hi team,

customer met an issue that in some specific situation, the input of the amplifier that is designed as follower (supplied by single 5V) would be -0.8VDC, and customer found the in this condition, the output of OPA2377 is zero but LM358 will be positive voltage. I did some simulation as attached. Could you pls help me understand the internal difference between OPA2377 and LM358 and the reason why it perform so different.

Besides, is there any risk for customer when the input is -0.8VDC since the abs is V(-)-0.5V?

opa2377 LM358.TSC

Thx~

Rayna

  • 0
    TI__Mastermind 23965 points

    Rayna, 

    The linear operating input common mode voltage of the OPA2377 is from the negative rail and 1.3V below the positive rail On a single supply this would be ground and 1.3V below the positive rail. Operating outside of this linear operating range will result in the output slamming into either the positive rail or ground. In a real measurement the OPA2377 can go to either rail depending on the sample that is picked. The abs max is 500mV beyond the supply rails. Operating in this range the device will not provide the desired results and operating beyond this range permanent damage may occur. If you can provide details on the application circuit and desired functionality we may be able to provide suggestions. 

    Best Regards, 

    Chris Featherstone

  • 0
    TI__Guru 64840 points

    Rayna,

    LM358 has bipolar PNP transistor input stage whereas OPA2377 is complementary CMOS transistor input stage.

    In order to prevent damage to PNP transistor, base to collector voltage cannot be greater than 300mV - see below Absolute Min input voltage of -0.3V

    Under the customer's condition the base to collector voltage would be 0.8V, which would most likely damage the junction - see below on the right.

    As far as OPA2377 goes, the Absolute Min voltage is 0.5V below negative rail (see below). Thus, in order to prevent damage the current needs to be limited to 10mA or less with added input resistor - see example above on the left.

    I have simulated the original conditions and condition with added 1k input resistor.  Only OPA2377 is fully protected with 1k resistor whereas VF5 input of LM358 still exceeds absolute max rated voltage and therefore may be damaged.

  • 0 in reply to Marek Lis
    TI__Intellectual 2215 points

    understood. Thanks Marek!